In recent years, an apparatus for transmitting power in a non-contact manner becomes popular. The power transmission apparatus includes a power transmitting device for transmitting power and a power receiving device for receiving the transmitted power, and transmits the power from the power transmitting device to the power receiving device in a non-contact manner.
The non-contact power transmission apparatus has a structure in which a primary coil of a transformer is placed on the power transmitting device side, and a secondary coil of the transformer is placed on the power receiving device side, and the power transmitting device and the power receiving device are required to be placed as closely as possible to each other. Thus, although power transmission in a non-contact manner can be performed, convenience is not much improved.
Besides, recently, a power transmission apparatus of a magnetic field resonance system is also known. In the magnetic field resonance system, power can be transmitted by combination of a resonant element (also called a resonance element) provided in a power transmitting device and including a coil and a capacitor, and a resonant element provided in a power receiving device and including a coil and a capacitor. The power receiving device includes a drive circuit to drive its own equipment, and a load circuit such as a charger of a secondary battery. In the magnetic field resonance system, when an equipment is placed on a charging stand, positioning is not severely required.
In power transmission to which the magnetic field resonance is applied, a one-piece voltage resonance type inverter is conceivable to be adopted as a structure in which power is transmitted by a relatively simple structure and which is suitable for a small equipment. As an example in which the one-piece voltage resonance is applied, there is an inverter apparatus to light a discharge lamp (see, for example, FIG. 8 of Patent Literature 1).
However, in the one-piece voltage resonance type, although the structure is simple, plural resonant systems exist in a circuit, and there is a problem that the resonant mode changes by load variation or the like, and the operation becomes unstable.
Patent Literature 1: JP-A-10-41081